1. Field of the Invention
The present invention relates to a disk drive system employing a disk surface stabilization mechanism, and, in particular, an optical disk drive for carrying out rotation drive of an optical disk which is an optical information recording medium in a form of sheet which has a flexibility, an optical information recording device, an optical information reproduction device, and a disk cartridge used therefor.
2. Description of the Related Art
Optical disks are demanded to store mass digitized data, as digitization of television broadcasting starts in recent years. A basic method of improving recording density on the optical disks is to reduce the diameter of a beam spot used for recording/reproduction information onto/from the optical disk.
For this reason, it is effective to shorten in wavelength of light used for recording/reproduction, and also, it is effective to enlarge numerical aperture NA of an object lens applied there. As for the wavelength of light, the wavelength of approximately 650 nm of red light is used on DVD while 780 nm of near infrared light is used on CD. Recently, a semiconductor laser of purple-blue light has been developed and it is expected that approximately 400 nm laser light will be practically used.
Moreover, the object lens for CD has less than 0.5 NA while the object lens for DVD has approximately 0.6 NA. It is demanded that the numeral aperture (NA) be enlarged further to 0.7 or more, from now on. However, enlarging NA of the object lens and shortening the wavelength of light may result in increase in influence of aberration in case the light applied is weakened. Therefore, the margin of tilt on the optical disk may decrease. Moreover, since the depth of focus becomes smaller by enlarging NA, it will be necessary to increase focus servo accuracy in the optical disk drive.
Furthermore, since the distance between the object lens and record surface of an optical disk becomes smaller by using the object lens of high NA, the object lens and optical disk may collide before focus servo control operation at the beginning thereof, unless surface vibration or axial runout on the disk is sufficiently controlled.
For example, as the O PLUS E (vol. 20 , No. 2) discloses on page 183 , as a large-capacity optical disk drive system having a short wavelength and high NA, a record film is formed on a rigid and thick substrate as in CD, and, light for recording/reproducing is not made to pass through the substrate, but recording/reproducing is made onto the record film through a thin cover layer is proposed.
Moreover, Japanese laid-open patent application No. 7-105657 and Japanese laid-open patent application No. 10-308059 disclose a method of stabilizing surface vibration on optical disk as a result of a flexible optical disk being rotated on a specially provided stabilization plate having a plane surface.
However, in case the substrate of optical disk is made of a rigid body, it is necessary to manufacture the optical disk at a very high accuracy and also to form the record film at a very low temperature condition in order to sufficiently reduce surface vibration and/or tilt of the disk which is rotating at high speed. Such requirements may reduce the yield of products, which may result in cost rise of optical disks.
Moreover, by the method for rotating an optical disk with flexibility on the stabilization plate, if it is made to rotate on a simple plane surface as disclosed by Japanese laid-open patent application No. 10-308059 , the optical disk and the stabilization plate may touch and slide. For this reason, the optical disk may vibrate, and surface vibration at high frequency may occur. Such a type of surface vibration at high frequency may fall in a frequency range for which mechanical focus servo control cannot deal with, and, thereby, residual servo error may not be sufficiently eliminated.
Furthermore, if the optical disk and object lens slide mutually due to surface vibration, dirt/dust may be generated thereby, which then may cause various error. Especially, as Japanese laid-open patent application No. 7-105657 discloses, the record film of an optical disk may be damaged in the case the record film is provided on the side facing the stabilization plate, which may directly result in recording/reproducing error.